The p53/miRNAs/Ccna2 pathway serves as a novel regulator of cellular senescence: Complement of the canonical p53/p21 pathway.

Aging is a multifactorial process characterized by the progressive deterioration of physiological functions. Among the multiple molecular mechanisms, microRNAs (miRNAs) have increasingly been implicated in the regulation of Aging process. However, the contribution of miRNAs to physiological Aging and the underlying mechanisms remain elusive. We herein performed high-throughput analysis using miRNA and mRNA microarray in the physiological Aging mouse, attempted to deepen into the understanding of the effects of miRNAs on Aging process at the "network" level. The data showed that various p53 responsive miRNAs, including miR-124, miR-34a and miR-29a/b/c, were up-regulated in Aging mouse compared with that in Young mouse. Further investigation unraveled that similar as miR-34a and miR-29, miR-124 significantly promoted cellular senescence. As expected, mRNA microarray and gene co-expression network analysis unveiled that the most down-regulated mRNAs were enriched in the regulatory pathways of cell proliferation. Fascinatingly, among these down-regulated mRNAs, Ccna2 stood out as a common target of several p53 responsive miRNAs (miR-124 and miR-29), which functioned as the antagonist of p21 in cell cycle regulation. Silencing of Ccna2 remarkably triggered the cellular senescence, while Ccna2 overexpression delayed cellular senescence and significantly reversed the senescence-induction effect of miR-124 and miR-29. Moreover, these p53 responsive miRNAs were significantly up-regulated during the senescence process of p21-deficient cells; overexpression of p53 responsive miRNAs or knockdown of Ccna2 evidently accelerated the cellular senescence in the absence of p21. Taken together, our data suggested that the p53/miRNAs/Ccna2 pathway might serve as a novel senescence modulator independent of p53/p21 pathway.

CCNA2 Ablation in Aged Mice Results in Abnormal rRNA Granule Accumulation in Hippocampus.

Mounting evidence in the literature suggests that RNA-RNA binding protein aggregations can disturb neuronal homeostasis and lead to symptoms associated with normal aging as well as dementia. The specific ablation of cyclin A2 in adult neurons results in neuronal polyribosome aggregations and learning and memory deficits. Detailed histologic and ultrastructural assays of aged mice revealed that post-mitotic hippocampal pyramidal neurons maintain cyclin A2 expression and that proliferative cells in the dentate subgranular zone express cyclin A2. Cyclin A2 loss early during neural development inhibited hippocampal development through canonical/cell-cycle mechanisms, including prolonged cell cycle timing in embryonic hippocampal progenitor cells. However, in mature neurons, cyclin A2 colocalized with dendritic rRNA. Cyclin A2 ablation in adult hippocampus resulted in decreased synaptic density in the hippocampus as well as in accumulation of rRNA granules in dendrite shafts. We conclude that cyclin A2 functions in a noncanonical/non-cell cycle regulatory role to maintain adult pyramidal neuron ribostasis.

Tissue-Specific Cell Cycle Indicator Reveals Unexpected Findings for Cardiac Myocyte Proliferation.

RATIONALE: Discerning cardiac myocyte cell cycle behavior is challenging owing to commingled cell types with higher proliferative activity. OBJECTIVE: To investigate cardiac myocyte cell cycle activity in development and the early postnatal period. METHODS AND RESULTS: To facilitate studies of cell type-specific proliferation, we have generated tissue-specific cell cycle indicator BAC transgenic mouse lines. Experiments using embryonic fibroblasts from CyclinA2-LacZ-floxed-EGFP, or CyclinA2-EGFP mice, demonstrated that CyclinA2-ßgal and CyclinA2-EGFP were expressed from mid-G1 to mid-M phase. Using Troponin T-Cre;CyclinA2-LacZ-EGFP mice, we examined cardiac myocyte cell cycle activity during embryogenesis and in the early postnatal period. Our data demonstrated that right ventricular cardiac myocytes exhibited reduced cell cycle activity relative to left ventricular cardiac myocytes in the immediate perinatal period. Additionally, in contrast to a recent report, we could find no evidence to support a burst of cardiac myocyte cell cycle activity at postnatal day 15. CONCLUSIONS: Our data highlight advantages of a cardiac myocyte-specific cell cycle reporter for studies of cardiac myocyte cell cycle regulation.

Mitotic events in cerebellar granule progenitor cells that expand cerebellar surface area are critical for normal cerebellar cortical lamination in mice.

Late embryonic and postnatal cerebellar folial surface area expansion promotes cerebellar cortical cytoarchitectural lamination. We developed a streamlined sampling scheme to generate unbiased estimates of murine cerebellar surface area and volume using stereologic principles. We demonstrate that, during the proliferative phase of the external granular layer (EGL) and folial surface area expansion, EGL thickness does not change and thus is a topological proxy for progenitor self-renewal. The topological constraints indicate that, during proliferative phases, migration out of the EGL is balanced by self-renewal. Progenitor self-renewal must, therefore, include mitotic events yielding 2 cells in the same layer to increase surface area (ß events) and mitotic events yielding 2 cells, with 1 cell in a superficial layer and 1 cell in a deeper layer (α events). As the cerebellum grows, therefore, ß events lie upstream of α events. Using a mathematical model constrained by the measurements of volume and surface area, we could quantify intermitotic times for ß events on a per-cell basis in postnatal mouse cerebellum. Furthermore, we found that loss of CCNA2, which decreases EGL proliferation and secondarily induces cerebellar cortical dyslamination, shows preserved α-type events. Thus, CCNA2-null cerebellar granule progenitor cells are capable of self-renewal of the EGL stem cell niche; this is concordant with prior findings of extensive apoptosis in CCNA2-null mice. Similar methodologies may provide another layer of depth to the interpretation of results from stereologic studies.